For the purposes of quality assurance, and also for monitoring industrial production processes, particularly in the field of precision engineering, optics, and in production engineering of optical, mechanical and electric microstructures, there is an increasing need for a precise measurement of work piece surfaces with the highest possible resolution.
Thus, for example, DE 10 2008 033 942 B3 has disclosed a distance sensor operating on the principle of multi-wavelength interferometry, which employs a plurality of laser light sources, the emitted wavelengths of which lie in the optical telecommunication range between 1520 nm and 1630 nm. The signals of the lasers used herein are combined in a common fiber by means of a multiplexer and transmitted to a multi-wavelength sensor head. In principle, such a multi-wavelength distance measuring method enables interferometric sensing of topologies and surfaces of any object using reflection geometry, wherein the multi-wavelength method provides a comparatively large, uniquely assignable measuring region and moreover renders it possible to achieve a measurement accuracy in the nanometer range and even in the sub-nanometer range.
Furthermore, DE 60 2004 004 916 T2 has disclosed an optical surface measuring device, in which a contour sensing distance sensor is placed substantially orthogonal to the surface to be measured. Here, the distance sensor is placed onto a rotatable device, which itself is arranged on a platform that is movable in relation to a measurement frame. Furthermore, a measurement surface is provided on the device receiving the distance sensor, the distance of which measurement surface to the measurement frame is measured by means of an apparatus for the contactless distance measurement.
In the case of such a sensor sensing the surface of an object in a contactless manner, for example within the scope of a scanning movement, the movement and the positioning accuracy of the sensor in relation to the object to be measured play a decisive role.
In order to be able to precisely establish the distance between the distance sensor and the surface to be measured, the sensor must be aligned substantially orthogonal to the surface to be measured and it must adapt the alignment thereof in accordance with the contour of the object to be measured. For this adaptation, both translational movements and rotational movements of the sensor are to be carried out.
In the case of a required measurement accuracy in the nanometer range or sub-nanometer range, a rotation of the sensor furthermore also always brings about a non-negligible translational displacement of the sensor in relation to the holder or platform supporting the sensor. Thus, the measurement signal from the sensor must be corrected for at least the positional displacement of the sensor caused by the rotational movement of the sensor. The mechanical tolerances of the sensor bearing cause non-reproducible positional changes of the sensor at different angular positions. It is therefore necessary to precisely determine the position of the sensor for every possible alignment of the sensor.
Currently known methods and devices for the highly precise measurement of surfaces have until now only been suitable for measuring surfaces having a predetermined symmetry. Sensing the entire surface of the object in this respect requires a rotation of the object, for example about the axis of symmetry thereof. By way of example, this allows the whole topology of the surface to be measured and logged by means of a measurement head or distance sensor mounted in a manner merely swivelable in a single plane. Such measurement concepts are not suitable for objects with a so-called free-form surface, which does not follow any predetermined symmetry.